This invention relates to a variable gain tilt control system for use with an optical amplifier, and more particularly with a rare earth doped optical fiber amplifier.
Optical amplifiers and particularly erbium doped optical fiber amplifiers are nearly ubiquitous in optical transmission systems, particularly in the field of telecommunications. Erbium doped fiber amplifiers (EDFAs) have high polarization insensitive gain, low cross talk between signals of different wavelengths, good saturation output power, and a noise figure close to the fundamental quantum limit. The excellent noise characteristics allow hundreds of these amplifiers to be cascaded to cover spans of thousands of kilometers of optical fibre. EDFAs as opposed to electronic repeaters are also transparent to data rate, signal format, and wavelength over a limited range, making them useful for wavelength multiplexed (WDM) communication systems that simultaneously transmit a large number of signals using different wavelength bands for each signal.
Notwithstanding these generally excellent characteristics, a disadvantage associated with EDFAs is their narrow spectral width and uneven gain band. The useful telecommunications window of an EDFA is approximately 20-30 nm wide, while an ideal amplifier would have a flat spectral gain across the full spectrum which extends from approximately 1520 nm to 1570 nm. The peak wavelength of the erbium gain spectrum varies from about 1530 nm to about 1535 nm depending upon the host glass material. FIG. 1 shows the characteristic gain spectrum of a particular conventional EDFA where it can be seen that the gain as a function of wavelength varies; this variation will be referred to hereinafter as gain ripple. Numerous techniques have been published for widening and flattening the gain spectrum (i.e. reducing the ripple) and include for example co-doping an erbium-doped silica glass fibre with Al2O3; changing the host glass material itself; using various forms of attenuating filters to reduce the gain at the emission peak; and, constructing hybrid devices having two or more different types of serially connected erbium doped fibre and actively adjusting pump conditions independently in each fibre section to compensate for the different gain slopes of each fibre.
In addition to the aforementioned problems and solutions associated with minimizing gain ripple, another significant problem exists to which there have been no simple, inexpensive, and practicable solutions. This other significant problem solved by this invention relates to improving dynamic gain tilt. The term dynamic gain tilt as used hereafter means the variation in gain at one wavelength as a result of changing the gain at any other wavelength via a change in input EDFA operating conditions. Although the techniques described above for minimizing gain ripple can provide a relatively flat spectrum in a specified wavelength band for a specific set of input optical powers and wavelength, the gain equalization performance degrades rapidly when the gain is changed (change in average population inversion levels) from the nominal conditions by changing the input power to the amplifier. One reported solution to this problem is allegedly achieved by a hybrid fibre device having cascaded amplifying stages with different gain spectra and an equal number of pump sources to allow the gain spectra of the individual stages to be effectively tuned independently so that when the total gain is changed, the relative contribution of each stage can be adjusted to arrive at the desired gain, with a resulting gain spectrum having a minimal amount of spectral distortion over the selected wavelength band. As an example, an erbium doped fibre having a positive gain slope may be combined with a different erbium doped fibre having a negative gain slope such that the hybrid device has a nearly flat gain at specific input power conditions. However, if the overall gain of the hybrid device must be changed, the gain slope of each of the constituent states will generally change at different rates when the pump power input to one of the stages is changed. In order to achieve good compensation at the new operating point, the relative gain of each of the constituent gain stages must be readjusted to make the gain slopes compensate each other. In implementing this type of amplifier, one skilled in the art would likely cascade two or more different erbium doped fibre compositions and provide a separate pump source for each amplifying stage at an end of each stage so as to minimize the number of splices and make it as convenient as possible to independently control the pump power to each stage. However, this technique for reducing or improving dynamic gain tilt requires a complex control scheme during operation in which the total power of multiple pump sources must be coordinated in order to realize gain slope compensation over a range of different gains (i.e. to change input power while maintaining a fixed target output power).
U.S. Pat. No. 5,764,406 in the name of Newhouse et al. entitled Hybrid Optical Amplifier Dynamic Gain Tilt incorporated herein by reference, describes a system wherein an erbium doped fibre amplifier device has a dynamic gain tilt that is less than the gain tilt of any of the constituent fibres. The hybrid device has at most one less pumping source than the number of constituent waveguides of the device. The hybrid device automatically provides a change in the pump distribution among the constituent doped waveguide sections so as to achieve a readjustment of the relative gains of the constituent sections. In one embodiment, this invention provides constituent EDFs of different co-dopant compositions that provides an automatic change in the pump distribution or partitioning among the constituent EDF sections so as to achieve a readjustment of the relative gains of the constituent EDF sections.
Although the ""406 patent appears to achieve its intended function, it is a relatively costly and is a complicated solution to dynamically controlling gain tilt.
Most known solutions for correcting for dynamic gain tilt have an associated power loss (approximately 5 dB) and furthermore, increased power is required for extra pumping. Another deleterious result of these systems is an increased resulting noise.
It is an object of this invention to provide a novel filter and method of use of such filter that can be placed in-line within an optical amplifier for varying the gain tilt dynamically as the power of the input signal is varied.
It is another object of the invention to provide an inexpensive filter for use in controlling tilt gain of an optical amplifier.
In accordance with the invention, there is provided, a variable slope optical filter for in-line use with an optical amplifier signal, the filter being disposed at an input side, an output side, or within the optical amplifier, the filter having a wavelength response that is substantially linear in slope within a band of operation wavelengths of the amplifier, the slope of the filter in said band of wavelengths being between zero and a only one of a positive or negative number, the filter for passing a band of wavelengths having a centre wavelength xcexc and having an amplitude response that has an opposite and counter slope as a function of wavelength to that of the amplifier""s gain tilt within the operation band of wavelengths; and,
a means for providing relative movement between the filter and the optical amplifier signal to vary the slope of the filter within the band of operation wavelengths.
It should be noted, that relative movement may be actual displacement of one of the filter and the input port or, alternatively, by varying the relative angle between the input port and the filter.
In accordance with the invention there is further provided, an optical amplifier having coupled thereto, an optical filter for dynamic and variable gain tilt control for use with the amplifier, the filter in a predetermined wavelength band having a width of at least 10 nm where gain tilt control is required, having a centre wavelength xcexc and an amplitude response that has an opposite and counter slope as a function of wavelength to that of the amplifier within the predetermined wavelength band; and,
a controller and movement mechanism for varying the slope of the filter in response to a control within the predetermined band, the output response of the filter having a slope throughout the predetermined wavelength band which is always only one of negative and positive.
In accordance with the invention there is further provided, an optical filter for dynamic and variable gain tilt control of the signal, the filter in a wavelength band of operation where tilt control is required, having a centre wavelength xcexc and an amplitude response that has slope of an opposite sign as a function of wavelength to that of the amplified optical signal prior to passing through the optical filter within the predetermined wavelength band; and,
means for relatively moving at least one of the beam and the filter to vary the slope of the filter in a substantially linear manner within the predetermined band as the beam passes therethrough, the output response of the filter having a slope throughout the predetermined wavelength band which only negative or positive.
In accordance with another aspect of the invention, a method for controlling gain tilt of an amplified beam is provided, comprising the steps of:
relatively moving at least one of a filter having two different regions and a beam to be filtered by the filter, each region of the filter having a different amplitude response as a function of wavelength, said relative movement being performed in a controlled manner to vary the power of the beam with wavelength, so as to correct for undesired gain tilt of the amplifier, at least one of the regions of the filter having a response that is substantially sloped to offset tilt, the filter being moved such that the beam propagates through portions of both regions of the filter, the proportion of the beam propagating through the two distinct regions being varied to dynamically correct for undesired gain tilt of the amplifier.